The invention relates to a method and a system for controlling a separator unit for multiphase separation of fluids of different densities, wherein either a pressure in the separator unit or a level of one or more of the liquids in the separator unit is adjusted in relation to a reference value.
Today, separators in connection with offshore oil and gas extraction traditionally are placed above water. However, for economical and extractional reasons, the development goes in the direction of a placement on or below the sea bed. The advantages herewith in principle will increase with an increasing ocean depth and an increasing distance from receiving plants for fluids from the separator, but simultaneously the complexity and costs associated with control and energy supply to the individual control devices will increase.
The control of the separator units for multiphase separation of fluids of different densities traditionally is carried out via complicated and resource-demanding, instrumented control loops. The conventional control loops normally will comprise measuring of a parameter value, and signal transmission to a regulator which compares the measured value with the reference value and decides the action which, in the form of a control signal, is sent back to the control device seeking to correct a possible deviation. The control device requires energy, something which for the traditional solutions involves an external supply via hydraulics, electric power or pneumatics. The most important regulating functions in connection with separation plants will be control of gas pressure and level control of liquids. Relevant control devices for example will be mains frequency controlled pumps and process control valves.
The level control systems which today are qualified for installation in an under-water separation plant, are relatively large and bulky, and in addition have a long response time. With increasing distances one will get an increased time delay. For relatively large separators one may be able to obtain a satisfactory stability of the respective parameters, even if the control loop is slow. For more compact separators the response time often will be significantly shorter, something which may turn out to be difficult to obtain with the qualified level measuring principles of today.
The main object of the invention is to provide a method and a system for achieving a simple, precise, robust and energy-economical mechanical control loop for pressure and level control of fluids in connection with different types of separators and tanks placed above or below sea level.
A more particular object of the invention is to provide a method and a system wherein the control loop for the process is substantially simpler and quicker than the control loops in separator plants according to the prior art, at the same time as external energy supply can be limited to possible signal lines to electronic pressure sensors and solenoids.
The above-mentioned objects are achieved with a method of the introductorily stated type which, according to the invention, is characterised in that the reference value and the relevant pressure in the separator unit, or the level of the relevant liquid converted to a pressure, are supplied to either side of a pressure sensitive mechanical means which moves with deviations of said pressure from the reference value, and that the movement is transferred directly to a mechanical control unit connected to a control device on a fluid outlet from the separator unit, and utilising the difference between said pressure in the separator unit and the pressure downstream of the control device for moving the device in the desired direction for correcting for the deviation.
According to the invention there is also provided a system of the introductorily stated type, which system is characterised in that it comprises a pressure sensitive mechanical means arranged to be supplied with the referent value and the relevant pressure in the separator unit, or the level of the relevant liquid converted to a pressure, to opposite sides of said means, and to be moved with deviations of said pressure from the reference value, and a mechanical control unit connected to the mechanical means and to a control device on a fluid outlet from the separator unit, and arranged to utilize the difference between said pressure in the separator unit and the pressure downstream of the control device for moving the device in the desired direction for correcting for the deviation.
In the method according to the invention there is established a reference value in the form of a pressure, and this reference value is compared with the actual pressure in the separator unit, or with the actual liquid level converted to a pressure. A possible deviation is converted directly to a mechanical movement which in turn is coupled to a mechanical control unit arranged to utilize the difference between the pressure of the separator chamber and the pressure downstream of the respective control device, to establish the force required by the control device in order to correct the deviation.
If the relevant well delivers gas, one will want to utilise the gas from the upper part of the separator tank as a driving medium for all the control devices. This is due to the circumstance that the gas will contain little contamination that may damage valve seals etc. It should otherwise be mentioned that the control devices will be able to be constructed in such a manner that they require a very small supply of driving medium as compared to the fluid flow controlled by the final controlling element. The driving medium therefore may be dumped into the process line downstream of the respective control devices without this having any practical influence on the separator functions.
It is necessary to construct the control devices such that a sufficient manipulated variable force is obtained also when the difference between the separator pressure and the pressure downstream of the relevant control device is at the lowest level, for example when a final controlling element in the form of a throttle valve is maximally open. This implies that one bases oneself upon pressure-balanced or little force-demanding valve devices, and that for possible other types of devices one provides for a sufficient manipulated variable force by increasing the surface influenced by the driving medium.
If special circumstances should dictate that it is not appropriate to use a fluid from the separator tank as a driving medium, one could alternatively utilise the pressure difference between a driving medium (e.g. hydraulic oil) supplied from the surface and the pressure downstream of the relevant control devices. This will enhance the price and complicate the operation of the separator, but one achieves the advantages of the invention in the sense that a deviation gives an approximately instantaneously correcting effect on the final controlling element because the control loop is based on a direct-acting mechanical connection.
Mechanised regulating processes according to the invention are based upon the actual value being compared with a reference value which is a combination of a spring tension and a pressure, the reference value being changed in that one or both of these parameters are changed. There will relatively seldom be a requirement to change the reference values for the liquid levels in the separator tank. It is more of interest to be able to change the reference value for the pressure in the separator tank in a simple manner. For practical reasons one will preferably choose to control this by means of electronic sensors. This is done by using a valve device which is arranged to change the pressure component of the reference value by means of a solenoid which alternatively provides for supply or discharge of fluid from a reference chamber in the device. The advantages of this method are that it is simple to change the pressure in the separator, at the same time as the external energy consumption for the regulation mainly is limited to a moderate current consumption for the operation of pressure sensors and solenoids. The technology for establishing such a reference pressure will be known to a person skilled in the art.